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Study Reveals Evidence of Multicentury Drought

Findings Show 3,000-Year Precipitation Record for SoCal, Tie to El Niño

In his research published today, Cal State Fullerton’s Matthew E. Kirby, who studies past climates, found that Southern California has experienced multicentury drought periods over the last 3,000 years.

With the state’s severe drought and growing concerns about meeting water needs next year and beyond, the study reveals that the region could face an even worse-case scenario of future drought. In fact, so-called megadroughts that last more than 30 years are short compared to what Kirby’s study has found.

“This is ground-breaking because our research shows conclusively that the variability in wetness and dryness has changed tremendously over the past 3,000 years, showing evidence of centennial-scale droughts — much longer than the epic or megadroughts identified from tree-ring studies,” said Kirby, associate professor of geological sciences.

Kirby is lead author of the paper, published in the journal Quaternary Science Reviews with co-corresponding author Sarah J. Feakins, assistant professor of earth sciences at USC. Other research collaborators include Cal State Fullerton geology graduate student Christine Kraft-Hiner, who will complete her studies in December, and CSUF alumna Joanna Fantozzi, who earned her master’s degree in geology in 2012.

The National Science Foundation-funded research received by Kirby and Feakins entailed collecting sediment from Zaca Lake, northwest of Santa Barbara in the Santa Ynez Mountains. The researchers studied the sediment’s various chemical and physical characteristics from below the lake’s bottom — dating back 3,000 years.

The remote natural lake provided a rare opportunity to reconstruct the frequency and severity of past precipitation variability beyond the historical period or tree-ring records, giving a picture of climate changes dating back thousands of years ago.

The study also compared the 3,000-year-old history of winter rainfall and dry periods with El Niño conditions, which confirmed a “clear connection” between El Niño and precipitation in Southern California. The findings also are the first to show that this relationship is robust for the region in the period before measured records, which document only the last 100 years or so, Kirby explained.

“It’s exciting because we’re providing a glimpse on how precipitation has changed, but more importantly, why it’s changed,” he said. “It demonstrates that the amount of rain in Southern California is predominantly responding to conditions in the tropical Pacific Ocean.”

This finding is particularly relevant this year as forecasts for El Niño have decreased to a 65 percent chance for this winter, indicating a potential extension of the state’s present drought, Kirby added.

“Not only does this study tell us that El Niño is the dominant force of winter precipitation in Southern California over the past 3,000 years, but it means that any changes to the tropical Pacific Ocean caused by global warming are likely to have a direct impact on our future water availability and subsequent crises.”

As a result, the study can help to plan for the future by understanding the history of water in the geologically recent past.

“Our goal was to understand the history of water in Southern California. It’s that simple,” Kirby said. “From my perspective, knowing how past water availability changed and why will also allow for better water management decisions in years to come.”